Method of determining traces of nitric in sulphuric acid solutions



Patented June 20, 1933 unrrn safes FEE STANLEY L. HANDFORTH, OF VTGODBURY, NEW JERSEY, ASSIGNOR TO E. I. DU PON'I DE NEMOURS & COMPANY, OF WILDIINGTON, DELAWARE, A CORPORATION OF DELA- WARE METHOD OF DETERMINING- TRACES. OF NITRIC IN SULPHURIG ACID SOLUTIONS No Drawing.

This invention relates to a method of determining small amounts of nitric acid in sulphuric acid and more particularly to a method of determining the degree of denitration of residual acid from denitrating spent or waste acid or from concentrating nitric acid by means of sulphuric acid or other dehydrating agent.

Spent or waste acid results from many chemical processes, and is in most part sulphuric acid, but contains small amounts of nitric acid. In order to reconcentrate the sulphuric acid for further use, it is necessary to remove the nitric acid substantially completely. Also, in concentrating weak nitric acid, the most common procedure is to mix it with strong sulphuric acid and to distill off the nitric. The sulphuric acid can then be reconcentrated and re-used. Since various types of equipment are used for the two steps, it is necessary to remove the nitric acid practically completely in the step of distilling oh the nitric and denitrating the residual sulphuric acid before it can be re-used. It is not desirable, however, to carry the boiling or denitration with steam beyond the desired point, as otherwise the nitric will be diluted with the excess water distilled from the sulphuric acid. No convenient continuous method has heretofore been devised for determining when the last traces of nitric acid have been removed from the sulphuric solution or other dehydrating agent. The well-known brown ring test can be made from time to time, but is not suitable as a continuous method of indicating or recording the amount of nitric present. Also in some cases the residual sulphuric acid is so dark in color as to make this test extremely doubtful.

I have now found that the amount of these small traces of nitric acid in the sulphuric acid may be determined not only continuously but accurately and definitely, by means of an electrical system. the E. M. F. developed between two dissimilar electrodes of particular types is very' greatly affected by, the amount of nitric contained in this sulphuric acid, whereas this E. M. F. is only slight-1y affected by changes I have found that Applicatioii filed January 26, 1932. Serial 1N0. 589,066.

instrength in the sulphuric acid, or in the temperature of the sulphuric acid within the usual limits of operation.

In carrying out my invention, I find it advantageous to place the two electrodes in the stream of acid issuing from the equipment in which the denitrating process is being-carried out. They may be placed in the stream either before or after cooling the acid. One elect-rode may be in the form of a pot or areceptacle, through which the stream of acid flows while the other may be inthe form of a plate or rod dipping in the liquid in the receptacle, or both electrodes may be plates or rods dipping in the stream of flowing acid, but separated slightly from each other. The E. M. F. developed between these two dissimilar electrodes is then determined by means of any desirable electrical apparatus, such as an indicating or recording voltmeter or potentiometer. While I found it possible to use several difierent pairs of electrodes, I prefer those pairs which comprise, for example, copper and high silicon iron alloy (5 to 15% silicon), and antimony and platinum, although either the anticurately small traces of nitric acid on the order of a few hundredths or down to a few thousandths of 1%. I have also found that where organic matter is present in the waste acid, such as in the acid resulting from the nitration of glycerin, 1f the denitratlon is carried too far and the temperature raised too hlgh, a charrmg of this organic matter results and this is indicated by a decrease in the E. 'M. F. developed between the two electrodes to below that developed in pure sulphuric acid. In fact, it may result even in a complete reversal of the polarity. It is thuspossible to continuously indicate or record the degree of denitration from overdenitration to under-denitration and to accordingly control the amount of heat supplied to the denitration process. Since under some conditions the electrodes may be very slightly corroded or worn away, it is sometimes desirable to arrange these so that they will be continuously fed in as this occurs. This may easily be accomplished in several ways. 7

It will be apparent from the foregoing that many various ways of carrying out this invention will suggest themselves and that many embodiments thereof exist and maybe practiced without departing from the spirit thereof. It is therefore understood that I do not intend to limit myself to the specific embodiments or illustrations described, except as'indicated in the appended patent '1 claims.

I claim: r v I 1. The method of determining the quan tity of nitric acid in a solution with other acids which comprises determining the E. M. F. developed between two dissimilar electrodes immersed in the solution.

2. The method of determining traces of a nitric acid in an acid solution, which com} prises determining the E. M. F develope'd between an inert electrode unaffected by nitric acid, and an electrode comprising a metal affected by nitric acid, which metal is taken-from Groups 1 and '5 of the periodic arrangement of elements. i

3. The method of claim 2 in Which the inert electrodeis composed of an alloy. A

4. Themethod of claim Qin'which the acid solution contains other compounds.

,5. Themethod of determining traces of nitric acid in sulphuric acid solutions, which comprises determining the E. ;M.,-F. developed between an electrode composed of one or more metals unaffected by nitric acid, and an electrode taken from a group consisting of copper, silver, and antimony. p I

6. The method of claim 5 in whichthe first named electrode is an alloy of metals unaffected by nitric acid.

7. The method of claim 5 in which the first named electrode is a metal taken from a group consisting of metals of the platinum group, and high chromium and silica irons. 8. The method of determining traces of nitric acid in sulphuric acid solutions, which comprises determining the E. M. F. developed between a copper electrode and a du'riron electrode immersed-in said solution.

9. The method of determining traces of nitric acid in sulphuric acid solutions, which comprises determining the E. M. F. developed between a platinum electrode and an Ztntimony electrode immersed in said soluion.

10. In the process'of denitrating a dehy i drating agent, the method of determining the degree of denitration which comprises de- 

